Conventional golf balls, solid or wound, typically have at least one core layer and at least one cover layer. Two-piece balls having a solid construction are popular with golfers because they provide a very durable ball with high initial velocity resulting in longer flight distance. Due to the rigidity of the materials used, however, the balls have a “hard” feel when struck with a club and a relatively low spin rate that makes them difficult to control, particularly on shorter approach shots.
Wound balls, i.e., spherical solid rubber or liquid center with a tensioned elastomeric thread wound thereon, are preferred by some golfers for a softer feel and higher spin enabling better control in and around the green. Wound balls typically travel a shorter distance, however, when struck as compared to a two piece ball. Moreover, as a result of their more complex structure, wound balls generally require a longer time to manufacture and are more expensive to produce than a conventional two piece ball.
Solid cores, used in wound or solid golf balls, are generally formed of a polybutadiene composition. In addition to one-piece cores, solid cores can also contain a number of outer layers, such as in a dual core golf ball. Covers, for solid or wound balls, are generally formed of ionomer resins, balata, or polyurethane, and can consist of a single layer or include one or more layers, such as a double cover having an inner and outer cover layer. The difference in play characteristics resulting from these different types of materials and constructions can be quite significant.
For example, ionomer-covered golf balls are typically harder than balata-covered golf balls, resulting in a more durable ball with a low spin rate. In contrast, balata-covered golf balls are less durable, but have a soft “feel” with high back spin for better control. Golf ball manufacturers have attempted to produce golf ball covers that provide the spin rate of balata with the cut resistance of an ionomer by forming various blends of materials. U.S. Pat. Nos. 4,884,814, 5,120,791, 5,324,783 and 5,492,972 disclose cover blends of high hardness and low hardness ionomers. However, none of the disclosed ionomer blends have resulted in the ideal balance of carrying distance, coefficient of restitution, spin rate and initial velocity that would approach the highly-desirable playability of balata-covered golf balls.
Other materials have been employed in golf ball covers in further attempts to provide a balata-like “feel” with an ionomer-like durability and distance. For example, polyurethane golf ball covers can be formulated to possess the softer “feel” of balata covered golf balls, such as disclosed in U.S. Pat. Nos. 3,147,324, 4,123,061, and 5,334,673. In particular, U.S. Pat. No. 5,334,673 discloses the use of two categories of polyurethane available on the market, i.e., thermoset and thermoplastic polyurethanes, for forming golf ball covers. Conventional golf ball covers made from polyurethane, however, have not fully matched ionomer-covered golf balls with respect to resilience or the rebound characteristics desirable to achieve the high initial velocity when stuck with a club.
In an attempt to provide golf balls that deliver the maximum performance in terms of both distance and spin rate for golfers of all skill levels, while still maintaining the desired aesthetic qualities discussed above, a number of golf ball manufacturers have introduced multilayer golf balls. The multilayer golf balls can include multiple cores, one or more intermediate layers, and one or more cover layers, wherein the layers can be formed of different or similar materials. U.S. Pat. No. 5,314,187 also relates to golf balls having a cover formed with multiple layers, wherein the outer layer, a blend of balata and elastomer, is molded over the ionomer resin inner layer. UK Patent Application Nos. GB 2,291,817 and 2,291,812 are both directed towards a wound golf ball with dual cover layers formed from balata or ionomer resins, wherein the inner cover layer has a high hardness as compared to the outer cover layer. U.S. Pat. No. 5,885,172 discloses a multilayer golf ball having an inner cover layer formed of a high flexural modulus material and a very thin outer cover layer formed of a castable, reactive liquid thermoset material. U.S. Pat. No. 6,210,283 discloses a double-layer cover using an ionomer inner cover and urethane outer cover.
Manufacturers have designed multilayer balls to have differences or similarities in the hardness of the layers of the ball to simulate the soft feel of balata, but still maintain the desirable properties of an ionomer resin cover. This difference in hardness can be accomplished through the use of substantially different materials in the different layers, or through the use of similar materials with various additives or differences in processing. For example, U.S. Pat. No. 6,132,324 discloses a multilayer golf ball having an high flexural modulus inner cover layer that is harder than the casted thermoset outer cover layer, while U.S. Pat. No. 4,431,193 relates to a multilayer cover having a hard, high flexural modulus ionomer resin inner layer and a soft, low flexural modulus ionomer resin. U.S. Pat. No. 6,117,025 discloses a three layer ball, wherein each layer has at least a three point difference in Shore D hardness and the intermediate layer is softer than at least one other layer of the ball. U.S. Pat. No. 6,126,559 discloses a soft core with a hard, thick cover of at least 60 Shore D in an attempt to provide a ball with distance and a comparable coefficient of restitution.
While the prior art has attempted to provide a golf ball having a soft feel, good spin, and distance through the use of a hard inner and soft outer cover, there exists a need in the art to provide such a ball using alternative methods with a dual cover or intermediate layer/cover combination, wherein the two layers have essentially the same hardness. There also exists a need in the art to provide a ball having two layers with the same hardness, but the two layers are different from each other with respect to specific processing or performance characteristics.